As is known, the short circuit strength of an IGBT (insulated gate bipolar transistor) is thermally limited with regard to its destruction after a turn-off. This can be attributed to the fact that a very high energy is liberated during a short circuit situation in the IGBT, which energy has to be dissipated rapidly by cooling if destruction of the IGBT is to be prevented.
At the present time, IGBTs are soldered on one side by means of solder by their chip rear side onto a so-called DCB ceramic (DCB=direct copper bonding). This involves a ceramic plate which is coated with copper on both sides and is used in modular technology as a mechanical carrier of the chips and for making electrical contact with the chips. It is the case, however, that solder has a relatively weak thermal conductivity. On account of this unfavorable thermal property, it thus represents a “brake” for the cooling of the IGBT.
On account of the low thermal conductivity of solder, the solder connecting technique is therefore occasionally replaced by the thermally better suited low-temperature connecting technique (LTC). The latter enables an increased dissipation of heat from a semiconductor chip.
Finally, it is also possible to cool a semiconductor chip from its front side in addition to its rear side. For this purpose, a further thermal capacity, which is made available by a relatively thick molybdenum lamina, for example, is applied to the front side of the chip.
To summarize, the following proposals have thus been made heretofore for improving the dissipation of heat from a semiconductor chip of an IGBT:
(a) replacing the customary solder on the rear side of the chip by LTC,
(b) soldering a lamina composed of molybdenum, in particular, onto the front side of the chip, and
(c) replacing the solder at the rear side of the chip by LTC as in proposal (a) and connecting a lamina composed of molybdenum, in particular, onto the front side of the chip by means of LTC.
However, at the present time, LTC is still not suitable for a modular production of IGBTs. Although soldering a lamina composed of molybdenum onto the front side of the chip boosts the dissipation of heat from the semiconductor chip, its effectiveness is limited owing to the poor thermal properties of the solder that is once again used in this case. There is not yet any practical experience for connecting a lamina composed of molybdenum by means of LTC.
For these reasons, none of the above proposals (a) to (c) has gained acceptance in practice heretofore.
The problem of rapidly dissipating heat from a semiconductor chip occurs not only in the case of IGBTs but also, in principle, in the case of diodes in surge current operation. The semiconductor chip is destroyed in this case, too, if the energy generated in surge current operation cannot be dissipated fast enough from the semiconductor chip. The above considerations for IGBTs therefore hold true in the same way for diodes.
During the fabrication of, in particular, IGBTs and diodes, a proton irradiation may be performed in order that deep dopant profiles or profiles with charge carriers having a reduced lifetime are incorporated into a semiconductor chip in a targeted manner. The fabrication of a field stop zone at a depth of 10 μm, for example, from the rear side of the chip by means of a proton irradiation shall be mentioned as an example. If, in this example, the proton irradiation is carried out from the rear side of the chip, then a strongly localized excessive increase in doping arises in the so-called “end-of-range region” of the proton implantation. This excessive increase in doping is then situated approximately at a distance of 10 μm from the rear side of the chip. FIG. 2 of the accompanying drawings show such a profile of the doping concentration as a function of the depth from the rear side of the chip using a solid line, working with an energy of 750 keV in silicon and with an effective proton dose of 1e12 cm.sup.−2 in this case.
The strongly localized excessive increase in doping is unfavorable for the switching behavior of IGBTs and diodes, however, since an electric field that penetrates into the field stop zone, that is to say into the region with the strongly localized excessive increase in doping, is abruptly braked, which leads to a great increase in the voltage rise dU/dt. This great increase in dU/dt poses considerable difficulties in applications of IGBTs and diodes.
For the switching behavior of IGBTs and diodes, it would therefore be significantly more favorable if the profile of the field stop zone, the so-called field stop profile, trailed off gently into the semiconductor chip. With a conventional proton implantation from the rear side of the chip, however, this is possible only when a plurality of implantations with different energies and thus different penetration depths are performed in a staggered manner. However, this constitutes a considerable outlay.
As an alternative, thought has already been given to carrying out a proton implantation from the front side of the chip. The “end-of-range region” would be greatly widened in this case. However, it is then necessary to radiate through the entire drift zone of the semiconductor component. As a result of this radiating-through process, however, on the one hand, the doping in the drift zone is determined by the so-called “tail region” of the proton irradiation precisely in the case of semiconductor substrates having a very high resistance. On the other hand, the position of the proton peaks relative to the rear side of the chip becomes dependent on fluctuations in the thickness of the semiconductor substrate. Moreover, properties of oxide layers on the front side of the semiconductor chip may also be altered by the irradiation.
Overall, then, proton irradiation both from the rear side of the chip and from the front side of the chip poses considerable problems that have not been solved heretofore.
For targeted relief of the loading of the edge of diodes or IBGTs during dynamic switching, the HDR concept (HDR=high dynamic robustness) employed at the present time seeks to reduce the plasma flooding in the edge region of diodes, in particular. This reduction of the plasma flooding in the edge region relieves the loading of the latter during dynamic switching operations. This relief of loading is actually realized by means of a masked cathode implantation into the rear side of the chip below the active chip region, but not below the edge region. In other words, an injection of charge carriers below the edge region scarcely takes place as a result of this. What is disadvantageous about such a procedure, however, is the need for a phototechnology on the rear side of the chip as well in order thus to be able to perform the masked cathode implantation below the active region but not below the edge.
If an irradiation technique is used for reducing the lifetime of the charge carriers in the edge region of a diode, then it is necessary, if a plurality of chips are present on a semiconductor wafer, also to apply a plurality of thick metal masks, the accurate alignment of which is very complicated.